1. Field of the Invention
The present invention relates to a motor drive circuit to drive a fuel pump of an automobile, for example. More particularly, this invention pertains to a drive circuit for a three-phase half-wave motor of a sensorless type having no sensor for detecting the rotational direction.
2. Description of the Related Art
Conventionally, driving a sensorless type three-phase half-wave motor is using a commutation circuit, which drives and controls the motor using voltages induced by motor coils.
FIG. 4 illustrates a drive circuit for a three-phase half-wave motor rotatable in two directions. This drive circuit includes a three-phase half-wave motor 30, a drive current output circuit 31, and a commutation circuit 32. In the motor 30, U-phase, V-phase, and W-phase motor coils 33, 34, and 35 each have one end connected to power supply V.sub.CC, and an other end connected to U-phase, V-phase, and W-phase output terminals of the drive current output circuit 31. In the commutation circuit 32, PNP transistors Q.sub.4, Q.sub.5, and Q.sub.6 have their bases and collectors connected together, and their emitters respectively connected to U-phase, V-phase, and W-phase output terminals of the drive current output circuit 31. Resistors R.sub.1 and R.sub.2 are connected in series between the commonly connected collectors of the PNP transistors Q.sub.4 to Q.sub.6 and the power supply V.sub.CC.
Resistors R.sub.3 and R.sub.4 each have one end connected to the V-phase and W-phase output terminals, and an other end connected to the emitter of a PNP transistor Q.sub.1. Resistors R.sub.5 and R.sub.6 each have one end respectively connected to the W-phase and U-phase output terminals and another end connected to the emitter of a PNP transistor Q.sub.2. Resistors R.sub.7 and R.sub.8 each have one end connected to the U-phase and V-phase output terminals, and another end connected to the emitter of a PNP transistor Q.sub.3. The transistors Q.sub.1 to Q.sub.3 have their bases connected to a series node A of the resistors R.sub.1 and R.sub.2, and their collectors connected to the U-phase, V-phase, and W-phase input terminals of the drive current output circuit 31, respectively.
The operation of the above motor drive circuit will now be explained referring to waveforms in FIG. 5. Given that the voltages induced by the motor coils rise in the order of U-phase to V-phase, then to W-phase as shown in FIG. 5A, when half the sum of the voltages induced by the V-phase and W-phase motor coils (the emitter voltage of the PNP transistor Q.sub.1) exceeds the voltage V.sub.a+V.sub.F (V.sub.a =the voltage at the node A, V.sub.F =the emitter-base voltage of the PNP transistors Q.sub.1 to Q.sub.3), the transistor Q.sub.1 is turned on and the U-phase motor coil 33 is excited. The voltage at the node A is the voltage difference between the collector voltage of the PNP transistors Q.sub.4 to Q.sub.6 and the voltage of the power supply V.sub.CC which is divided by the resistors R.sub.1 and R.sub.2. The collector voltage of the PNP transistors Q.sub.4 to Q.sub.6 is the combined voltage of the three-phase induced voltages minus the emitter-base voltage of the PNP transistors Q.sub.4 to Q.sub.6. Assuming that the resistance of the resistor R.sub.2 is "0" and that the emitter-base voltage V.sub.F of the PNP transistors Q.sub.1 to Q.sub.3 is considered negligible, the U-phase, V-phase, and W-phase motor coils are driven at timings differing by 120.degree. from one another as shown in FIGS. 5B, 5C, and 5D, thereby rotating the motor.
With the conventional three-phase half-wave motor drive circuit, however, the motor coils are excited independent of the direction, forward or reverse, in which the motor rotates. For example, the sum of the voltages induced by the V-phase and W-phase motor coils 34 and 35 is used to drive the U-phase motor coil 33. Whichever motor coil 34 or 35 rises first, the emitter voltage of the PNP transistor Q.sub.1 will have the same waveform. If the waveform shown in FIG. 5A corresponds to a motor rotating in the forward direction, when the motor coil voltages rise in the order of W-phase to V-phase, then to U-phase, the motor will rotate in the reverse direction.
At the beginning of activation, the three-phase half-wave motor of this type is rotatable in either direction (forward or reverse direction), in accordance with the relationship between the positions of multiple permanent magnets, provided on a rotor (not shown), and the positions of magnetic fields produced by the motor coils. The above-described conventional circuit to drive such a three-phase half-wave motor excites the motor coils independent of the direction the motor turns, forward or reverse, at the beginning of activation. In other words, the motor sometimes turns in the forward direction, and sometimes in the reverse direction, and it is not possible to restrict the rotational direction of the motor to either the forward or reverse direction.